Fix: .do files are not run from do/ dirs

It should produce exactly the same result whether a .do file is under
a do/ dir or not (e.g. ./do/y.do or ./y.do), but it wasn't the case, for
example:

$ cat ./do/default.do # empty file
$ cat ./do/x.do
redo y
$ redo y # builds ./y
redo  y
$ redo x # builds ./x and ./do/y
redo  x
redo    do/y

The last command should build ./x and ./y, not ./do/y.